In vitro screens employing newly excysted, 6- and 12-week-old flukes, in a medium permitting the linear growth of the parasites, were assessed. When exposed to certain known fasciolicides, newly excysted flukes were susceptible only to diamphenethide, the free amine of diamphenethide, emetine hydrochloride and albendazole. Older flukes were affected by a much wider range of compounds including the chlorinated hydrocarbons, the substituted phenols and the salicylanilides. However their susceptibility to diamphenethide and its active metabolite was decreased significantly. The activity of fasciolicides in these in vitro assays therefore closely parallels their activity in vivo. When several broad spectrum anti-nematode agents were evaluated against newly excysted flukes in these screens the benzimidazole, isothiocyanate, pyrimidine and imidazothiazole anthelmintics showed activity but 12 potent antiprotozoal agents were all inactive. It is concluded that these in vitro assays were useful for detecting any intrinsic activity that a compound might possess against flukes. Such activity could often be missed in conventional in vivo screens because of problems associated with host pharmacokinetics. Negative results from such in vivo screens could preclude the development of more bioavailable derivatives or pro-drugs as novel and useful fasciolicidal agents.

The response of the free-living stages of Nippostrongylus brasiliensis, Nematospiroides dubius, Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia ostertagi to a wide variety of antiparasitic agents in vitro was investigated.All the major broad spectrum veterinary anthelmintics showed good activity against each of these worms with EC30 values varying from about 00002mg/1 for certain benzimidazoles and ivermectin to about 6–5 mg/1 for febantel. Of 22 known narrow spectrum anthelmintics useful only against H. contortus and/or helminths other than trichostrongyles, only 10% showed good activity at concentrations equal to or less than 100mg/1. Further, only one of 15 antiprotozoal agents showed good activity in these tests at the 100mg/1 level.

Approximately 60% of fourth-stage larvae of Nematospiroides dubius recovered from mice 6 days after infection, developed to the young adult stage when cultured over a 7-day period in a complex medium in vitro. Larvae at the late fourth stage of development were found to be highly susceptible to most broad spectrum anthelmintics under in vitro conditions, the benzimidazole, imidazothiazole, pyrimidine, isothiocyanate and macrocyclic lactone compounds all being active at very low concentrations. Narrow spectrum anthelmintics active only against ascarids, pinworms, filariae, cestodes or trematodes had little or no effect on these larvae. Ineffective also were those chlorinated hydrocarbon, substituted phenol and salicylanilide compounds known to affect Haemonchus but not trichostrongylid worms in general. It is concluded that in vitro assays employing larvae of N. dubius are useful for the stringent screening of compounds for broad spectrum antitrichostrongyle activity. Used in conjunction with in vivo screens employing N. dubius in mice, they also afford means for detecting intrinsic activity against the parasite in a system free from any complicating host pharmacokinetics.